Altitude or distance from the ground is the primary variable that affects the ground sampling distance (GSD) of the images. The GSD in the overlapping areas between the missions should not differ by more than a factor of two. So for the same project, captured with the same camera, the highest flight height at which images are taken should not exceed two times the lowest flight height:
GSD1 ≤ 2 * GSD2
(Fr * Iw) / (Sw * H1 * 100) ≤ 2 * (Fr * Iw) / (Sw * H2 * 100)
H1 ≤ 2 * H2
- GSD = Ground Sampling Distance [cm/pixel].
- Sw = sensor width [mm].
- H = flight height [m].
- Fr = real focal length [mm].
- Iw = image width [pixel].
It is usually recommended to process images captured at the same flight height, as they have the same Ground Sampling Distance (GSD). It means that all images will have the same level of detail. This facilitates the matching of keypoints between images and therefore, helps the reconstruction.
For more information on how to capture the images for terrain with height variations: Image acquisition plan for terrain with height variations.